Both gels in Question 19 were run for the same duration at the same voltage. The left lane of each gel was from the exact same sample. Band X (labeled on the left gel) also showed up on the right gel, but it traveled less than half the distance of band X on the left gel. Briefly, what does this information tell you about the gel on the right compared to the gel on the left

Mg (s)       +    HCl (aq)    →  MgCl₂(s)   + H₂(g)

Looking at the equation :

We have 1 Mg at the left hand side and 1 Mg as well on the right hand side.
So that is balanced. 

We have 1 H at the left hand side and 2 H on the right hand side.
So that is not balanced.  Same for Chlorine. Cl.

We add 2 to the HCl on the left hand side and that balances it.

Mg(s)       +    2HCl(aq)    →  MgCl₂(s)  + H₂(g)